Ambulatory monitoring of blood or tissue oxygen saturation in implantable medical devices is generally limited to monitoring trends of uncalibrated oxygen saturation measurements over relatively short periods of time. The influence of body motion, optical path length, sensor location, and the relationship of an uncalibrated oxygen saturation index to the physiological status of the tissue, e.g. to actual tissue oxygenation, can result in a broad statistical distribution of the response of an uncalibrated oxygen saturation index to patient conditions. Pulse oximeters are used for bedside monitoring of a calibrated measure of hemoglobin oxygen saturation using absorbance measurements of red an infrared light. External, non-invasive devices are available which use fiber optic light sources for monitoring tissue oxygen saturation based on the absorbance of near-infrared light by hemoglobin and myoglobin. However, chronic ambulatory monitoring of tissue oxygen availability in a blood-perfused tissue would be useful for monitoring a patient condition. A need remains for a sensor capable of monitoring calibrated tissue oxygen saturation and total hemoglobin volume fraction that is reduced in size and power requirements for use in an implantable or wearable medical device for ambulatory patient monitoring.